JP3358135B2 - High strength steel excellent in sulfide stress cracking resistance and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a steel used for high-strength steel pipes for oil wells having a yield strength of 84 to 100 kgf / mm ² and excellent in sulfide stress cracking resistance, shaped steel, and various other members. And a method of manufacturing the same.

[0002]

2. Description of the Related Art As shown in Japanese Patent Publication No. 56-33459, a steel excellent in sulfide stress cracking resistance is produced by quenching and tempering a steel containing a specific chemical component by a usual method. It has been known. Although there is no description of the crystal grain size in such an invention, it is empirically estimated that the number is 6 to 9. Also, ISIJ International vol. 32 (1992)
In p1021, it is also known that making the crystal grains fine improves the resistance to sulfide stress cracking.

[0003]

However, the steel having excellent sulfide stress cracking resistance obtained by the prior art has a yield strength of not more than 80 kgf / mm ² and an increase in strength (yield strength of 84 kgf / mm ² or less). Up to 100 kgf / mm ² ), while maintaining the sulfide stress cracking resistance, which rapidly degrades, the strength is further improved by the conventional quenching and quenching.
It was impossible with tempering.

The present invention solves the above-mentioned problems, and utilizes ultrafine grains not used in the conventional quenching and tempering treatments, and specifies the process by which these can be obtained and the steel components that can effectively utilize them. Accordingly, an object of the present invention is to provide a steel having extremely high strength (yield strength of 80 to 100 kgf / mm ² ) and excellent sulfide stress cracking resistance.

[0005]

Means for Solving the Problems The inventors of the present invention have conducted a number of experimental studies in order to achieve the above object, and as a result, have adjusted the steel composition and changed the crystal grains to the fine-grained steel generally used. Of 9
It has been found that by making the grain size even smaller than that, it is possible to obtain a steel pipe for oil wells having excellent sulfide stress cracking resistance that cannot be predicted from the extension of the conventionally known effect of crystal grain refinement.
However, such fine grains cannot be obtained by a general heat treatment method, and furthermore, when the grain size is reduced, the hardenability of the steel is significantly reduced. Therefore, it is necessary to devise a method of obtaining fine grains and securing the hardenability. Was. That is, the present invention has been made based on these findings, and its gist is that (1) C:
0.10 to 0.35%, Si: 0.01 to 0.50%,
Mn: 0.10 to 0.6%, S: 0.005% or less,
P: 0.015% or less, Mo: 0.30 to 1.0%,
Al: 0.005 to 0.1%, Nb: 0.01 to 0.1
%, Ti: 0.005 to 0.04% and Ti ≧ 3.
4N, N: 0.006% or less, B: 0.0008-
0.0016%, and optionally Cr:
0.1-1.5%, V: 0.01-0.1%, Co:
0.05-0.5%, Zr: 0.001-0.1% Rare earth element: 0.001-0.05%, Ca: 0.00
1 to 0.02% of one or two kinds, the balance being substantially composed of Fe, and having a particle size of 9.5 (ASTM No.)
It is a high-strength steel excellent in sulfide stress cracking resistance characterized by exhibiting a finer grain tempered martensite structure. Here, the crystal grain size means a grain size in an austenite state before transformation into martensite, that is, a prior austenite grain size.

(2) C: 0.10 to 0.35 by mass%
%, Si: 0.01 to 0.50%, Mn: 0.10 to 0.1%
0.6%, S: 0.005% or less, P: 0.015
% Or less, Mo: 0.30 to 1.0%, Al: 0.005
-0.1%, Nb: 0.01-0.1%, Ti: 0.0
0.5 to 0.04% and Ti ≧ 3.4N, N: 0.
006% or less, B: 0.0008 to 0.0016%, and if necessary, Cr: 0.1 to 1.5
%, V: 0.01-0.1%, Co: 0.05-0.
5%, Zr: 0.001 to 0.1% Rare earth element: 0.001 to 0.05%, Ca: 0.00
A steel containing 1 to 0.02% of one or two kinds and a balance substantially composed of Fe is heated to a temperature range of 930 to 1000 ° C. at a heating rate of 3 ° C./sec or more for 5 minutes or less. A high strength with a yield strength of 84 to 100 kgf / mm ² and excellent resistance to sulfide stress cracking, characterized in that the steel is reheated for a short time, then quenched to a martensitic structure, and then tempered below the A _c1 transformation point. Steel production method.

[0007]

The production method of the present invention will be described below in detail. First, the reason why the present invention is limited to the above steel components will be described. C is a basic component of steel that has the effect of increasing the strength of the steel and increasing the hardenability, but if it is too small, it has no effect, and if it is too large, it causes quenching cracking. %. Si has a deoxidizing agent remaining, but if it is too small, it has no effect, and if it is too large, the resistance to sulfide stress cracking decreases, so the content of Si is set to 0.01 to 0.50%.

[0008] Mn is essential for detoxifying S as a basic component of steel and is an effective element in terms of enhancing hardenability, but Mn itself significantly reduces sulfide stress cracking resistance. Therefore, the addition amount is set to 0.1 to 0.6%. S is an unavoidable impurity in steel, but forms MnS and is harmful to sulfide stress cracking resistance.
It was as follows. P segregates at the grain boundaries and greatly reduces sulfide stress cracking resistance.
% Or less.

Mo has the effect of increasing the resistance to sulfide stress cracking and has the effect of further improving the hardenability.
If it is less than 0.3%, the effect is not sufficient. If it exceeds 0.8%, the effect is not only saturated, but also Mo ₂ C precipitates and the toughness is deteriorated. -0.8%. Al has a deoxidizing agent remaining like Si. If the amount is too small, the effect is not obtained. If the amount is too large, inclusions increase and the resistance to sulfide stress cracking deteriorates.
005 to 0.1%.

[0010] Nb is the most important element for making crystal grains finer than 9.5. If the amount is too small, the effect is not obtained, and if the amount is too large, the effect is saturated and the cost is very high. Ti fixes N, which is a component that cannot be completely removed from steel, as TiN and exerts an effect of improving the hardenability of B. In particular, if the heating rate during reheating for quenching is high as in the present invention, Al
N is an essential element because N is not effectively formed. Further, it has the effect of forming Ti oxide to cause P to be present in the grains, thereby reducing the amount of segregation to the grain boundaries. If the amount is too small, this effect is not exhibited, and addition of 3.4N or more in the same atomic molar ratio is necessary for fixing N. However, if the content is too large, a large amount of TiC precipitates during tempering and significantly impairs the toughness. Therefore, the content was made 0.005 to 0.04% and Ti ≧ 3.4N.

[0011] N is inevitably contained in steel, but if it is too large, the hardenability of B may be impaired even if the added amount of Ti is increased. It is well known that B is an element that significantly improves the hardenability in a very small amount, but the hardenability does not decrease even if it becomes finer. In addition, the resistance to sulfide stress cracking does not decrease. On the other hand, the quenchability improving element except Mo has an effect of lowering sulfide stress cracking resistance. Therefore, B, which is extremely effective for changing the structure after quenching to martensite and does not reduce the resistance to sulfide stress cracking, is an essential element in the present invention. Particularly, it is an indispensable element in the steel of the present invention utilizing ultrafine grains. If the amount is too small, the effect is not sufficient, and Mo
In the case of the added steel, the effect of improving the hardenability is reduced even if it is too large, so the amount of addition is made 0.0008 to 0.0016%.

[0012] The steel having the above-mentioned composition has Cr, V, C
o, Zr, rare earth elements, and Ca are selectively added as needed. Cr is effective in improving hardenability and forming martensite, but when added in a large amount, it reduces sulfide stress cracking resistance. 0.
If it is less than 1%, there is no effect of addition, and if it exceeds 1.5%, the sulfide stress cracking resistance is remarkably reduced.
The amount of r added was set to 0.1 to 1.5%. V is effective as a strengthening element, but has no effect in a very small amount, and if added in a large amount, toughness is deteriorated. Therefore, V was added in an amount of 0.01 to 0.1%. Co has the effect of improving the sulfide stress cracking resistance by forming a robust film on the steel surface in a wet hydrogen sulfide environment and reducing the amount of hydrogen intrusion. The effect is not remarkable when the amount is small, and the effect is saturated even if added in a large amount, and the element is expensive. Therefore, the addition amount is set to 0.05 to 0.5%. Zr
Has the effect of forming a Zr oxide to cause P to be present in the grains, thereby reducing the amount of segregation at the grain boundaries. If the amount is too small, this effect is not exhibited, and if added in a large amount, there is a concern that a large amount of Zr oxide is formed and may be a starting point of cracking.
The amount was 001 to 0.1%.

[0013] Rare earth elements and Ca are effective elements that make the form of inclusions spherical and harmless. When the amount is too small, the effect is not obtained. When the amount is too large, the number of inclusions increases and the resistance to sulfide stress cracking decreases.
0.05%, 0.001 to 0.02%.

The steel having such a chemical composition must have a tempered martensite structure with a grain size smaller than 9.5. The factor that has the greatest effect on sulfide stress cracking resistance is the texture, which is one of the most important requirements of the present invention. Such high-strength steel is manufactured by quenching. However, if bainite other than martensite is mixed in the quenched structure, the resistance to sulfide stress cracking is reduced. Although a higher martensite ratio is better, a martensite structure of 95% or more, which is an industrially producible range, is herein used. Since martensite is too strong and very brittle, it is tempered to obtain the desired strength. Therefore, the target structure of the present invention is tempered martensite.

Next, the reasons for limiting the crystal grain size will be described.
Fig. 1 shows the change of the crystal grain size by the change of heating rate and heating temperature in steel of the same chemical composition, the martensitic structure by quenching, tempering at various temperatures, and the resistance to sulfide stress cracking. This is a result of measuring and examining the strength level at which the resistance starts to decrease suddenly. As can be seen from the figure, in the case of No. 9 or less obtained by normal quenching, the crystal grains become finer and the limit strength gradually increased. However, when the grain becomes finer than No. 9, the limit strength moves to a higher strength side than the extension line. Heretofore, the effect of such fine graining has not been known, and since it is generally difficult to obtain such fine grains, there has been no idea of using ultrafine grains. However, in order to overcome the conventional strength limit, such a fine grain structure is necessary. Therefore, the required particle size was made finer than 9.5.

The quenching method for obtaining such fine-grained martensite for the above steel will be described below. As quenching conditions, it is necessary to ensure fine grains and high quenchability. FIG. 2 shows the relationship between the heating rate and the crystal grain size. As can be seen from the figure, if the average heating rate is lower than 3 ° C./sec, the desired fine grains cannot be obtained. . When the heating time exceeds 5 minutes, grain growth starts, so the heating time was set to a maximum of 5 minutes. Fine grains are obtained as the heating temperature is lower, but in steels to which Mo and B are added in combination, if the heating temperature is too low, the hardenability decreases. In addition, when the heating temperature is increased, not only the grain growth becomes remarkable, but also the hardenability decreases again. FIG. 3 shows the relationship between the quenching property and the grain size and the quenching temperature. VC _-90 is a critical cooling rate at which a 90% martensite structure ratio can be obtained, and a smaller value indicates higher hardenability. As shown in the figure, the heating temperature was 930 to 1000 ° C. as a fine grain and high hardenability.

[0017]

EXAMPLES First, a steel having the chemical composition shown in Table 1 was melted and cast in a normal manner, and then a seamless steel pipe was manufactured by a model rolling mill.
Quenching and tempering were performed. The sulfide stress cracking test was performed on a material having a desired strength range while changing the tempering temperature. The test was performed by mixing 1N acetic acid and 1 mol of sodium acetate to obtain a pH of 10%.
In a liquid saturated with hydrogen sulfide + 90% nitrogen, a tensile stress equivalent to 80% of the yield strength was applied to a smooth round bar test piece, and the breaking time was measured. The test was performed up to 720 hours, and those that did not break can be regarded as having excellent sulfide stress cracking resistance. Tables 2 and 3 show the results.

Since the martensite ratio is difficult to measure after tempering, after quenching, a structural test piece was sampled and examined. The grain size is determined by J
It was measured based on the IS cutting method.

[0019]

[Table 1]

[0020]

[Table 2]

[0021]

[Table 3]

[0022]

As described above, according to the present invention, a steel pipe for an oil well which has been subjected to a heat treatment under specific conditions by specifying a steel component has high strength and excellent resistance to sulfide stress cracking. Have.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between austenitic grain size and critical strength at which sulfide stress cracking resistance is excellent.

FIG. 2 is a diagram showing a relationship between an acceleration speed and a crystal grain size.

FIG. 3 is a graph showing the relationship between hardenability, crystal grain size, and hardening temperature.

Continuation of the front page (56) References JP-A-4-21718 (JP, A) JP-A-62-253720 (JP, A) JP-A-63-238242 (JP, A) JP-A-63-4046 (JP, A) JP-A-52-66815 (JP, A) JP-A-1-56821 (JP, A) JP-A-63-105921 (JP, A) Patent 2579904 (JP, B2) (58) Int.Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (12)

(57) [Claims] C: 0.10 to 0.35% by mass, Si: 0.01 to 0.50%, Mn: 0.10 to 0.6%, S: 0.005% or less, P : 0.015% or less, Mo: 0.30 to 1.0%, Al: 0.005 to 0.1%, Nb: 0.01 to 0.1%, Ti: 0.005 to 0.04% And Ti ≧ 3.4
N, N: 0.006% or less, B: 0.0008 to 0.0016%, the balance being substantially Fe, and a particle size of 9.5.
High-strength steel excellent in sulfide stress cracking resistance characterized by exhibiting a finer grained tempered martensite structure than ASTM No. 2. The steel according to claim 1, further comprising Cr: 0.1 to
High strength steel excellent in sulfide stress crack resistance, characterized by containing 1.5%. 3. The steel according to claim 1, further comprising:
A high-strength steel excellent in sulfide stress cracking resistance characterized by containing 0.01 to 0.1%. 4. The steel according to claim 1, further comprising Co: 0.05 to 0.5%, wherein the steel further comprises Co: 0.05 to 0.5%. Strength steel. 5. The steel according to claim 1, further comprising 0.001 to 0.1% of Zr, having excellent resistance to sulfide stress cracking. High strength steel. 6. The steel according to claim 1, further comprising a rare earth element: 0.001 to 0.0.
A high-strength steel excellent in sulfide stress cracking resistance, characterized by containing one or two kinds of 5% and Ca: 0.001 to 0.02%. 7. In mass%, C: 0.10 to 0.35%, Si: 0.01 to 0.50%, Mn: 0.10 to 0.6%, S: 0.005% or less, P : 0.015% or less, Mo: 0.30 to 1.0%, Al: 0.005 to 0.1%, Nb: 0.01 to 0.1%, Ti: 0.005 to 0.04% And Ti ≧ 3.4
A steel containing N, N: 0.006% or less, B: 0.0008 to 0.0016%, and having a chemical composition substantially consisting of Fe, with the balance being substantially 930 to 1000 at a heating rate of 3 ° C./sec or more. Sulfide stress with a yield strength of 84 to 100 kgf / mm ² , characterized by being reheated to a temperature range of 5 ° C. for a short time of 5 minutes or less, then quenched to form a martensite structure, and then tempered at a transformation temperature of A _c1 or less. A method for producing high-strength steel with excellent crack resistance. 8. The steel according to claim 7, further comprising Cr: 0.1 to
The yield strength characterized by containing 1.5% is 84-
A method for producing high-strength steel excellent in resistance to sulfide stress cracking of 100 kgf / mm ² . 9. The steel according to claim 7, further comprising:
A method for producing a high-strength steel having a yield strength of 84 to 100 kgf / mm ² and excellent sulfide stress cracking resistance, characterized by containing 0.01 to 0.1%. 10. The steel according to claim 7, further comprising Co: 0.05 to 0.5%, characterized by a yield strength of 84 to 100 kgf / mm ² . Method for producing high strength steel with excellent sulfide stress cracking resistance. 11. The steel according to claim 7, further comprising 0.001 to 0.1% of Zr: a yield strength of 84 to 100 kgf / mm.
^2. Method for producing high-strength steel with excellent sulfide stress cracking resistance. 12. The method according to claim 7, 8, 9, 10, or 11.
The rare earth element: 0.001 to
A yield strength of 84 to 1 characterized by containing one or two of 0.05% and Ca: 0.001 to 0.02%.
A method for producing a high-strength steel excellent in sulfide stress cracking resistance of 00 kgf / mm ² .